This invention relates to dental restorations, and more particularly, to a method and apparatus for securing a dental post within a prepared endodontically treated tooth in connection with a coronal replacement using a post and cement techniques.
One of the most commonly used dental restorations is the formation of a crown to replace a missing coronal tooth structure. Such techniques generally involve formation of a bore, typically along the apical canal and the securement of a post within the bore. A core is built up onto the post head protruding from the bore, and an artificial crown is then secured in place on the core.
Various types of dental posts have been utilized for maintaining the superstructure in place. One type of post, generally referred to as active post, uses external threads for threading the post into the tooth understructure. While these active posts provide good retention, they generally increase the amount of stress in the tooth understructure and may cause cracking of the tooth. Additionally, even these active posts use cement in addition to the screw threads to retain the post in place.
Other types of posts are referred to as passive posts. While these include grooves to aid in the retention, they rely upon cement to secure the post in place. These passive posts generally do not provide the stress of the active posts. However, in order to provide good retention, adequate cementing techniques are required.
Numerous cements have been known to provide good to excellent results. Some of the more common cements that have been used include those of zinc phosphate, glass-ionomer cement, polycarboxylate, zinc oxide, and resins. One of the most common forms of the resin cement is a Bis-GMA resin. In addition, acrylic base cements have also been utilized such as a methacrylate.
In addition to relying upon the cement alone, additional techniques have been introduced to increase the retention with the available cements. A most common approach is to roughen the post space with a threader, or the like. Occasionally, this technique is referred to as providing a "corrugated" canal to enhance the cementation process.
U.S. Pat. No. 4,645,457 issued to Goldman and Nathanson, provides yet a further method for enhancing the cementing technique. This patent teaches that after forming the bore, there are dentinal tubules existing within the tooth understructure. However, there exists a sludge or smear layer that covers a portion of the bore or canal walls. This appears as an amorphous layer on the canal wall that obstructs the dentinal tubules. This patent, accordingly, teaches the use of a specific sequence of steps in order to prepare the tooth for a coronal replacement and achieve enhanced retention. The patent teaches a first step of flushing the prepared area with a chelating agent selected from the group consisting of citric acid and ethylenediaminetetraacetic acid (EDTA). This material serves to remove the debris and the smear layer. Thereafter, in a second step, which must follow the previous step, a solvent selected from the group consisting of sodium hypochlorite (NaOCl), surface active agents, and emulsifiers are used. This serves to open up the tubules. Thereafter, a suitable luting agent is utilized to secure the post in place.
Subsequent to the above identified patent, a continuation of this patent issued as U.S. Pat. No. 4,758,163 which covered a kit for applying these materials. A subsequent U.S. Pat. No. 4,850,872 issued also on this technique applying the technique to other types of restorations, including placing filler material in a tooth having a caries cavity.
The above mentioned technique covered by the patent, has been licensed to Roydent Company and the preferred material utilized in the commercially available product includes the use of EDTA and NaOCl. The cement that is used is an unfilled Bis-GMA resin. The cement has also been referred to as the Boston Post cement.
While various tests have been conducted to determine which of the various cements provide the best retention, in a study by Anthony H.L. Tjan, et al., in the Journal of Prosthetic Dentistry, September 1987, Volume 58, #3 in an article entitled "Effects of Various Cementation Methods on the Retention of Prefabricated Posts", it would appear that the best type of retention can be obtained by roughening the post hole prior to utilization of a cement. Furthermore, the type of cement that this article found to be most effective was a resin cement which was used after roughening the post holes using a tap. With such technique, the retention shown was an average of about 120 lbs. Using the technique described in the aforementioned patent, namely, providing initially a rinse of EDTA followed by an irrigation of NaOCl, and thereafter, use of the resin, the retention was an average of only approximately 55 lbs.
The type of resin that was used in this article was a highly filled resin which was selected because it has a lower polymerization shrinkage and higher mechanical strength than unfilled resin. Generally, the more filled the resin is, the lower the polymerization shrinkage. Accordingly, for this reason a high amount of filler was utilized in order to get a lower polymerization shrinkage.
The particular resin described in this article was a composite resin available from Sci-Pharm, Inc. in Duarte, Calif., available under the brand name of Support P/L.
While this Journal article pointed out that improved retention can be achieved through roughening the dental walls, such roughening can apply additional stress to the post canal, can weaken the wall, and adds additional time and complexity to the cementing process.
It would, accordingly, be beneficial if improved cementing techniques could be achieved whereby retention can be increased without the necessity of roughening or corrugating the canal walls.